CDMA (Code Division Multiple Access) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, for example spectral efficiency and the simplicity of frequency planning. An example of a known CDMA system is the broadband cellular radio standard EIA/TIA IS-95.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a pseudo-random sequence or a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate pseudo-random sequence is used over each connection between a base station and a mobile station, and the signals of the different users can be distinguished from one another in the receivers on the basis of the pseudo-random sequence of each user.
Matched filters provided in the receivers are synchronized with a desired signal, which is recognized on the basis of the pseudo-random sequence. The data signal is restored in the receiver to the original band by multiplying it again by the same pseudo-random sequence that was used during the transmitting stage. Signals multiplied by some other pseudo-random sequence do not correlate in an ideal case and are therefore not restored to the narrow band. They appear thus as noise with respect to the desired signal. The pseudo-random sequences or spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
CDMA systems generally employ so-called short codes that have typically the same length as the symbols to be transmitted and that are also constant in time, i.e. each symbol of the signal to be transmitted is multiplied with the same spreading code. When short codes are used, some advantages have been achieved especially in connection with the multiuser detection (MUD) and interference cancellation methods. In the MUD methods, the signals of several users are detected simultaneously by utilizing all the received information in the detection of the signals. The parameters used in the known MUD algorithms, particularly the so-called correlation matrix that contains the mutual correlations of the received signal components, are constant when using short codes and therefore they do not have to be estimated symbol by symbol. The drawback of the short codes, however, is that their number is limited, which results in the shortage of useful codes having good correlation properties for example during a handover or in a multirate system.
A multirate system refers to a system where the user signals can have different types of bit or symbol rates, depending on the service used. For example speech and data services may require different transmission rates. In prior art arrangements a multirate system is supposed to be implemented with methods of code division or subcode modulation. The former method means that a user requiring a greater data rate is given a set of codes of the length of a symbol and they are transmitted on parallel physical channels, each modulated with a different information symbol. In the latter method the user is given a set of codes having the length of a part of a symbol. In the subcode modulation it can be considered that the bit rate grows but the bit rate or chip rate of the spreading code and the allocated code remain constant. According to the method, the subcodes having the length of a part of a symbol are modulated, in which case the properties of the subcodes can generally no longer be controlled.
A so-called R-CDMA method where subsets of a long code are modulated is previously known. The prior art also includes methods of applying orthogonal and biorthogonal coding methods, for example the reference Comparison of very low rate coding methods for CDMA radio communications system by K. Rikkinen (IEEE ISSTA Proceedings, Jul. 4-6, 1994, University of Oulu, Finland), pp. 268-272.
The drawback of the above-described known methods is that the code control is lost or the number of codes is not sufficient. A method of the R-CDMA type provides a large number of codes for use, but the drawback is the uncontrolled randomness of a single code in the properties of the codes.